1. Field of the Invention
The present invention is in the field of navigation and/or location devices wherein an output of the device is the location of the device.
2. Description of Related Art
The desire to determine position, direction, and location is as ancient as human's inhabitation of the Earth. Over the course of many centuries, various systems have been developed to locate people and items with varied degrees of accuracy. To name a few, these navigation techniques include the compass, celestial navigation, GPS, and inertial navigation. Although many approaches have been considered, there is no simple, highly accurate methodology of automatic location and orientation tracking indoors at the appropriate level of sub-foot resolution required for indoor navigation applications. The present invention describes a novel method and apparatus for accurately ranging, locating, and tracking people or objects, optionally, indoors, with high precision. The invention is further benefited from its inherent compatibility and use of components already installed in many of today's mobile electronic devices and widely understood.
Probably the best known techniques for ranging, location and tracking use the time difference of arrival techniques in order to compute distances from a set of known transmitter. One example of this technique is arguably the most well known navigation system—a collection of satellites known as the Global Positioning System. The GPS systems use the speed of light as the reference to compute distance from known transmitter locations—satellites. However, it is widely known that the GPS system is unable to deliver highly accurate results indoors due to both signal attenuation and multi-path of RF signals in indoor environments. It is extremely difficult to achieve less than 3 m accuracy with a typical GPS receiver in an indoor environment. Additionally, as with other time of flight techniques, the receiver must be able to receive signals from at least 3 transmitters, in this case GPS satellites, in order to obtain a 3D position lock; this significantly reduces the availability of a solution and requires more infrastructure, e.g., satellites.
Another time of flight location system is the use of acoustic or ultrasonic beacons. These signals have the advantage of travelling slower, i.e., the speed of sound, so the time resolution required for accurate location determination is reduced. However, they also suffer from multi-path, echoes, interference from other audio or ultrasonic noise sources, e.g., fluorescent lights in the case of ultrasonic. Additionally, acoustic and ultrasonic signals are easily blocked. This means if an object is placed between the transmitter and the receiver, the receiver's ability to determine location is greatly reduced or extinguished. Again as with the GPS system, 3 transmitters are required to establish a 3D position solution. Because of the ease of blocking ultrasonic and acoustic signals, this makes reliable usage indoors with many walls and objects very problematic.
Another set of techniques employs signal strength instead of time of flight to approximate range. An example of such system is the use of signal strength from Wi-Fi access points or cellular base stations to determine location. Although signal strength from a radio transmitter such as RF is a function of distance, it is highly non-linear and unpredictable. It, therefore, is most frequently used as a coarse proximity detection of the receiver, typically to accuracies substantially lower than that of GPS. To achieve high accuracies, the use of many redundant RF transmitters or local RSSI signal mapping is required.
An additional shortcoming of both RSSI and RF-time-of-flight techniques is that no information about orientation or direction of received is contained in the transmitter signal. In order to determine orientation the use of multiple receivers co-located on the object at a separation distance is required. The accuracy of the orientation is directly related to the distance of separation, making accurate operation require significant physical separation of the receivers, which is not convenient on a small device.
There are other methods proposed and used for location determination, including the use of a camera, range finders and image processing algorithms to observe the local environment. However, these systems are complex and require user intervention, such as holding a camera or permanently affixing a camera to a helmet.
The present invention uses a fundamentally different mechanism for location determination. Instead of using time-of-flight measurement from three fixed points, this invention discloses a vector gradient, magnetic field as a transmitter or beacon, and a magnetic sensor located in proximity as a receiver. This technique has numerous advantages. As magnetic fields follow well understood and predictable gradients, precise range can be determined to sub-foot level. In addition, because the field is a vector field and the field has a distinct polarity based on position relative to the transmitter, orientation information can be extracted from the signal. In fact one transmitter can produce multiple axes of magnetic field, allowing a receiver to fully determine its 3D position and 3D orientation when in range of only one beacon as opposed to three or more. This is a significant advantage versus all of the aforementioned systems.
Due to the substantially higher accuracy of the disclosed system as compared to a the currently used techniques, a vector gradient method can be effectively utilized in conjunction with inertial navigation techniques to achieve accurate initialization and recalibration updates necessary to make inertial navigation practical in an indoor environment. This also enables a receiver to maintain accurate location and orientation information when out of range of a transmitter using hybrid inertial navigation techniques further described in the invention.
Lastly, without impacting the location accuracy performance of the disclosed system, magnetic field transmitter signals can be modulated, in frequency or amplitude or use other coding techniques, to transmit information to a receiver. Transmitted information can include a transmitter ID, transmitter location, encryption keys, local information such as on-site services available, and information to aid inertial navigation techniques for example a reference number to an online map, or the local barometric pressure for correction of altitude errors. Transmitted information further enhances the utility and ease-of-configuration of disclosed systems. A need exists for a low-cost location device with high accuracy and extended capabilities.